1. Field of the Invention
The present invention is directed to a method for the amplification of input signals of a hearing aid, wherein a compression of the signals picked up by the hearing aid ensues dependent of the acquirable signal level. The present invention is also directed to a circuit for the implementation of the aforementioned method.
2. Description of the Prior Art
There is usually an impaired sensitivity to volume given a hearing disorder of the inner ear. Soft signals are usually not perceived and must therefore be correspondingly amplified in the hearing aid, but loud signals are heard just as loud as in the case of normal hearing. Compression circuits are employed in hearing aids in order to adapt the amplification of the signals to the auditory sensation of a person with normal hearing. Such compression circuits contain an AGC (Automatic Gain Control) circuit that varies the amplification dependent on the signal level. Such AGC circuits have either an exactly defined behavior or can be modified by parameter settings (for example, compression ratio, cut-in point of the compression, time constant). These settings are determined in the individual adjustment of the hearing aid to the hearing-impaired person and are usually not changed during use of the hearing aid. Known compression algorithms implement either a level-limiting compression or a "full dynamic range" compression. In the level-limiting compression, the cut-in point of the compression usually lies at a relatively high level, at which level the signals are significantly compressed (high compression ratio). As a result, loud signals are prevented from being too highly amplified. In "full dynamic range compression", by contrast, the cut-in point of the compression usually lies as low as possible, with the compression ratio being selected such that the loudness sensed by the hearing-impaired person is balanced over the entire level range. An input-output characteristic that is based on stationary signals forms the basis for known techniques for calculating the parameter settings of a dynamics compression. Particularly in the employment of a loudness scaling, the sensed loudness is determined for stationary signals and the required compensation by the hearing aid is calculated therefrom. Given chronologically variable, modulated signals, i.e., speech, however, the time behavior of the compression circuit leads to an effectively different compression behavior. As a result the loudness perceived by the hearing-impaired person cannot be optimally compensated for all signals.